Fiber lasers have significant advantages over traditional lasers, including stability of alignment, scalability, and high optical power of a nearly diffraction limited output beam. In a fiber laser, the gain medium is a length of an optical fiber, the core of which is doped with an active lasing material, typically ions of a rare earth element, such as erbium or ytterbium or both. The lasing material is usually pumped using an emission of a diode laser, or an array of diode lasers. The advent of double clad active optical fibers having inner and outer claddings, in which the pump light is coupled to the inner cladding to be absorbed at the doped fiber core along the fiber length, allowed a considerable increase in overall output power of a fiber laser, while preserving the brightness and directivity of a singlemode output laser beam. Power levels of the order of several kilowatts or even tens of kilowatts in an almost singlemode output laser beam are now achievable, opening a great variety of industrial applications, such as concrete drilling or sheet metal cutting for a car industry or shipbuilding.
At high optical power levels of fiber lasers, the task of managing stray light becomes crucial. A doped fused silica core and a fused silica inner cladding or claddings of the fiber lasers are surrounded by an external coating made of a polymer. Having an external polymer coating is essential because without it, the optical fiber becomes very brittle; furthermore, for some fibers called “polymer-clad fibers”, the polymer layer functions as an outer optical cladding. At high pump power levels, even a small fraction of stray light can heat the polymer to a temperature at which it can be damaged, causing catastrophic failure of the active fiber of the laser. For instance, in fiber laser arrangements where the fiber is pumped at one end, and a catastrophic thermal failure occurs at the other end, the fiber can actually start burning towards the pump end, causing the entire length of expensive active double-clad fiber to be eliminated.
In fiber lasers, the stray light and associated heating is caused by so called cladding modes, that is, modes of light propagation in the cladding. In double clad fibers, the cladding modes of the inner cladding are used to deliver the pump light to the fiber core. When the light of the cladding modes escapes the inner cladding, it can cause a localized heating of the fiber polymer coating, resulting in a catastrophic failure of the active fiber. Because of this, the cladding modes need to be removed (stripped) from the fiber where they are no longer required, or where they should not be normally present, such as in outer cladding of a double clad fiber. For example, when an active optical fiber is pumped at one end, the residual inner cladding light can be removed at the other end of the fiber to prevent its further propagation. Furthermore, the cladding modes present in the outermost cladding can be removed at the pump end of the active fiber. The cladding light can include the residual (unabsorbed) pump light, amplified spontaneous emission (ASE) of the active fiber core, and the laser light at the wavelength of lasing that escaped the fiber core.
Cladding modes are removed using so called cladding mode stripper devices, or cladding mode strippers. A cladding mode stripper of the prior art has a layer of a high-index material disposed next to and optically coupled to the cladding of the optical fiber. The cladding light present in the cladding is coupled to the high-index material and is absorbed in the high-index material or in an opaque solid shield disposed around the high-index material. An index-matching gel or a coating of a high-index polymer is typically used in a cladding mode stripper. By way of example, Wilhelmson et al. in U.S. Pat. No. 4,678,273, which is incorporated herein by reference, disclose a mode stripper having a radiation-resistant high-index material surrounding the optical fiber.
To facilitate a more uniform distribution of cladding mode light stripped along a length of an optical fiber, the refractive index of the high-index polymer can be made to vary along the fiber. For example, Wetter et al. in an article entitled “High power cladding light strippers”, Photonics West 2008, Fiber Lasers V: Technology, Systems, and Applications, Proc. of SPIE Vol. 6873, 687327, which is incorporated herein by reference, discloses a high-power cladding mode stripper having the refractive index varying along the fiber length. Anderegg et al. in U.S. Pat. No. 7,349,596, which is incorporated herein by reference, disclose a cladding mode stripper device in which a polymer with negative temperature dependence of the refractive index is deposited along the fiber. The negative temperature dependence of the polymer limits the local cladding mode stripping effect when the polymer is locally heated to a high enough temperature. The cladding modes can be stripped off by the cooler part of the coating disposed downstream of the optical fiber; as a result, the “hot spots” in the cladding mode stripper device are avoided and the temperature distribution evens out.
Jürgensen in U.S. Pat. No. 6,999,481, which is incorporated herein by reference, discloses a cladding mode stripper device in which the sheath (the outer coating) is gradually thinned along the fiber so that the cladding modes can escape gradually, thus lowering the peak temperatures. Hu et al. in US Patent Application Publication 2008/0131060 A1, which is incorporated herein by reference, disclose a cladding mode stripper, in which a light-scattering material is deposited on the fiber to scatter the cladding mode light. Furthermore, Frith in US Patent Application Publication 2009/0080835 A1, which is incorporated herein by reference, discloses a “gradual” cladding mode stripper for a multi-cladding fiber, in which the claddings of the fiber are removed one by one in a step-like fashion, the high-index material being placed at the steps where the claddings are removed, to couple the cladding modes out. Disadvantageously, the prior-art approaches are not scalable to very high optical power levels, being specific to particular fiber types and particular optical power ranges.
The prior art lacks a cladding mode stripper device that would be simple yet scalable to high optical power levels. Accordingly, the present invention provides such a device.